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Full text of "The manufacture and properties of iron and steel"

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18                                           INTRODUCTION.
order that fhere shall be sufficient work upon the steel to give it a proper physical structure, and these heavy ingots mean a larger cross-section, and this means that it takes a longer time for the ingot to cool from the liquid to the solid state.
During all the time the ingot is liquid there is a process going on by which the carbon, the phosphorus, and the sulphur are becoming concentrated in the central portion of the mass and rising to the upper portion. During the operation of rolling and shearing off the ends, the worst of the ingot is discarded, but the central portion of what is left is not uniform with the outside portions.' It is evident that in most sections this impure portion will constitute the neutral axis, and thus its influence be reduced to a minimum. In certain cases, however, as in armor plate and ordnance, great care is taken to reject all contaminated portions. This could be done in structural material, but it would involve much expense, and no engineer would be justified in insisting upon such a course, since contracts are founded upon ordinary commercial practice, and this ordinary practice allows a certain measure of segregation to exist. Specifications are sometimes written in which explicit directions are given that in tests cut from the finished material an increase will be permitted in the allowable content of impurities. This is simply stating clearly what has long been a recognized fact.
Perhaps the most troublesome instances of segregation occur in plates rolled directly from ingots. It usually happens that the top surface of the ingot is solid and that a cavity exists beneath. When this is rolled into a plate, it is possible to shear the plate so that this inner cavity is not opened, and we then have a finished plate which has an area of lamination and an area of segregation, and these are not in the center of the plate, but near one edge. The test pieces are almost always taken from the corners, so that they never reach the segregated portion, and there is nothing to mark the dangerous condition of the plate. In plates rolled from slabs there is often a streak of segregation running through the central axis, but there is not the centralization of impurities that occurs in the older method of manufacture.
When an ingot of steel is cast in a mold and allowed to cool it is not a homogeneous mass of uniform strength throughout. Its